building on an out-of-tree directory fails

packaging this for Debian's pkg-xorg with the usual defaults, like using a separate 'build/' directory, fails:

libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I../../../../../src/GLdispatch/mapi/vnd-glapi -I../
../../.. -I../../../../../include -I../../../../../src/GLdispatch/mapi -I../../../../src/GLdisp
atch/mapi -I../../../../src/GLdispatch/mapi/glapi -I../../../../../src/GLdispatch/mesa -DMAPI_M
ODE_GLAPI -DMAPI_ABI_HEADER=\"vnd-glapi/glapi_mapi_tmp.h\" -D_FORTIFY_SOURCE=2 -g -O2 -fstack-p
rotector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall -Werror -std=gnu99 -i
nclude config.h -fvisibility=hidden -DUSE_X86_64_ASM -c g_glapi_inittable.c  -fPIC -DPIC -o .li
bs/g_glapi_inittable.o
g_glapi_inittable.c:53:19: fatal error: glapi.h: No such file or directory
 #include "glapi.h"
                   ^
compilation terminated.